The present invention relates generally to the field of distillation devices and, more particularly, to such devices which are adapted for low volume on-site solvent recovery operations.
Reclaiming of highly flammable solvents, such as naptha, acetone, etc. by distillation procedures has been a common practice for a number of years. Such operations however, have traditionally occurred in rather large, complex facilities suited to high volumes and continuous operations. While occasionally such large facilities are justified by the volume of reclaimable spent solvents generated by on-site industrial or commercial processes, quite often the reclaimable spent solvents are transported from a large number of relatively small generators. Because of the nature of such operations, personnel involved in the operation of such facilities are skilled by virtue of previous training or experience.
Unfortunately, due to their complexity and high maintenance cost, large scale facilities, such as referred to above, are not adaptable to relatively low volume applications. As a result, there has been recent interest in developing relative low cost, portable, low volume solvent recovery distillation units for on-site reclamation. Previous designs for such units incorporate an evaporation vessel, a condensing heat exchanger and a storage vessel. The condensing heat exchanger usually comprises a coaxial tube arrangement having the cold fluid jacket surrounding the vapor communicating pipe connecting the two vessels. The rate of evaporation in such exchangers is directly related to the increase in total surface area of condensation. Obviously, for a given volume, this surface area is maximized by employing a relatively small diameter pipe which is suitably coiled to increase the total length of the pipe. Unfortunately, use of a relatively small diameter pipe results in a flow restriction and increased pressure differential along the length of the pipe. In previous distillation units of this type, the decrease in distillation rate caused by this flow restriction is compensated for by heating the spent solvent at an increased temperature. This is not energy efficient, however, and increased operating temperatures increase the risks of combustion.
The following patent references disclose distillation units specifically designed for such use:
______________________________________ U.S. Pat. No. Inventor ______________________________________ 4,457,805 Pastor 4,323,429 Hoover ______________________________________
U.S. Pat. No. 4,457,805 to Pastor discloses a solvent recovery device which employs a double wall tank with an electrical heating element. The heating element heats a low viscosity heating liquid disposed between the double walls of the tank. Spent solvent is contained within a removable plastic bag disposed within the tank. Purified solvent vapors created by evaporation within the tank are routed outside the tank to a double walled heat exchanger where the solvent vapors are condensed. Purified liquid solvent then gravitates to a reservoir separate from the tank. The present invention differs from the Pastor device in several important respects. For example, the Pastor device employs electrical controls which by their mere presence increase the risk of explosion of the highly flammable solvent. Further, the plastic bag in the Pastor device is an inherently inefficient medium for transferring heat from the heating liquid to the spent solvent. The present invention avoids the use of an insulating material, such as a plastic bag, between the tank wall and spent solvent. Additionally, the Pastor device employs a coiled tube double wall heat exchanger and reservoir external of the waste holding tank to condense the solvent vapors. Not only does this arrangement add to the complexity and bulkiness of the overall unit, but it also unnecessarily restricts the condensation rate of the purified solvent vapors.
U.S. Pat. No. 4,323,429 to Hoover discloses a spent solvent purification apparatus which is similar to Pastor in that it employs an electrical heating element and control system and separate evaporating and storage vessels connected by a condensing heat exchanger of the coiled tube type. Additionally, it is noted that neither the Pastor nor Hoover devices include a vacuum pump which is operable to lower the atmospheric pressure inside the evaporation and condensing units.